Image projection systems are important communication tools. They permit the conversion of digital or analog information into visual images that can be viewed by audiences as small as one and as large as several thousand.
While there are a wide variety of image projection systems varying from simple slide projectors to Imax theaters, most such systems use high intensity discharge lamps (HID lamps) to project an image. Unfortunately, HID lamps present serious problems in use.
A typical HID lamp comprises a pair of electrodes in a glass bulb (typically fused quartz) filled with metallic vapor. A voltage applied between the electrodes creates an arc of rapidly moving electrons between the electrodes. When the electrons collide with atoms of the metallic vapor, the collisions temporarily excite the atoms, and when the atoms drop back to their unexcited states, they emit light. Because of the ease with which mercury can be vaporized, it is widely used in HID lamps. To obtain an adequate density of vapor atoms, the mercury must typically be heated to relatively high temperature (hundreds of degrees Centigrade) and high pressure (hundreds of PSI). Indeed bulb surface temperatures in HID lamps can reach 590° C., and the lamps can reach pressures of 450 PSI.
The heat and pressure in HID bulbs present a number of operational difficulties. The heat presents risks of fire, burn injury to users and heat damage to nearby components. The pressure presents a risk of bulb explosion and a consequent spewing of hot fragments and toxic mercury. Achieving the necessary heat takes time, preventing a projector from rapid display, and once the needed heat is achieved, the exterior of the bulb and the surrounding equipment must be cooled with fans. The bulbs and the cooling fans reduce system reliability, and the noise of the fan is unacceptable for many applications such as home theaters.
Accordingly, there is a need for improved light sources for image projection systems and for image projection systems that can employ such improved sources.